1. Field of the Invention
This invention relates to a photomask material and particularly to a photomask material to be used for manufacturing a semiconductor device.
2. Description of the Prior Art
As a mask to be used for manufacturing a semiconductor device, a photograghic emulsion plate formed by a glass substrate was generally utilized in the early days. However, according to the tendency toward high integration with fine patterns, hard masks including a metallic thin film of a material such as chromium (Cr) formed on a transparent glass substrate are widely utilized these days (for example, see Japanese Patent Laying-Open Gazette No. 157247/1982 or No. 157249/1982).
FIG. 2 is a sectional view of a conventional photomask material. In the figure, a metal film 2 of chromium or the like is formed on a transparent glass substrate 1 of quartz or the like. This metal film 2 of Cr or the like is formed to a thickness of approximately 600 to 800 .ANG. on the transparent glass substrate 1 by an evaporation or sputtering process. In order that a photomask for semiconductor device may be obtained, photoresist or resist for an electron beam (referred to hereinafter as EB) is coated on the metal film 2 and patterns are formed by irradiation of light or EB, and after that, a developing process and an etching process etc. are applied. If the metal film 2 is formed of Cr, etching is applied by using ceric antimony nitrate and perchloric acid in a wet process or a mixed gas of carbon tetrachloride (CCl.sub.4) and oxygen (O.sub.2) in a dry process. In manufacturing of a mask for a semiconductor device, particularly a highly integrated device having fine patterns such as VLSI, a dry etching process which produces little side etching is preferred.
Although a wet etching process is generally adopted for manufacturing a Cr mask to be used as a mask for manufacturing a conventional semiconductor device, it is difficult in a wet etching process to obtain a mask of high precision due to a side etching effect and the like, while in a dry etching process, the etching speed of Cr is lower than approximately 100 .ANG./min and a ratio of selection of resist is not appropriate and, therefore, the dry etching process is not suited for mass production of photomasks. In addition, in the case of a Cr mask, adhesion to the quartz substrate is not good, causing fine patterns to peel off at the time of rinsing the mask.
As one solution to the above stated problems, a method might be considered in which a metal silicide film formed by a silicide transition metal of molybdenum (Mo), tantalum (Ta), tungsten (W) or the like is used as a mask material (for example, see Japanese Patent Application No. 61372/1984). More specifically, silicon (Si) contained in the quartz glass substrate and silicon (Si) contained in the metal silicide film as a mask material are effectively combined to produce strong adhesion. As for etching, dry etching can be easily done (at an etching speed of 1000 .ANG./min) by using a mixed gas plasma containing carbon tetrafluoride (CF.sub.4) and oxygen (O.sub.2), compared with the case of a chromium (Cr) mask.
However, the above stated transition metal silicide film has a light reflection factor as high as approximately 50% and as a result, at the time of pattern printing, the resolution of the patterns is decreased due to diverse scattering of light between the wafer and the mask, which makes it difficult to manufacture a VLSI device having submicron patterns.